A step of manufacturing an electrode which constitutes a secondary battery includes a step of applying a paste for the electrode on the surface of a collector sheet, and a step of drying the paste (i.e., removing a solvent). A hot-gas drying furnace is generally used for drying the paste.
The step of drying the paste using the hot-gas drying furnace is roughly as follows: a collector sheet having a surface to which the paste is applied is conveyed into the hot-gas drying furnace by web handling. Then, a hot gas is blown onto the paste on the collector sheet through a nozzle inside the hot-gas drying furnace, and thereby a solvent (e.g., NMP (N-methyl pyrolidone), or water) contained in the paste is evaporated to remove the solvent.
The term “remove” as used herein includes reducing a residual amount of a solvent to a predetermined standard value or less, and does not mean only removing a solvent completely.
It has been known that, in the case of drying the paste using the hot-gas drying furnace in this manner, the temperature of a hot gas is reduced by evaporative latent heat generated when a solvent is evaporated.
In an electrode for a secondary battery, a standard value regarding a residual amount of a solvent after drying is set. However, it has been known that the residual amount is increased when the temperature of the hot gas decreases.
In a conventional hot-gas drying furnace, the amount of a hot gas is increased to limit an influence of decrease in the temperature caused by evaporative latent heat, thus ensuring a drying condition where a residual amount of a solvent becomes equal to or smaller than a standard value.
However, when the amount of a hot gas (i.e., the amount of introduced outside gas) in the hot-gas drying furnace is increased, it is also necessary to increase the amount of a hot gas to be discharged from the hot-gas drying furnace.
The increase in the amount of introduced outside gas directly affects the increase in an air conditioning load in an area where the hot-gas drying furnace is installed. Therefore, conventionally, a running cost of the hot-gas drying furnace is excessively high and this increase in the running cost is a factor in an impediment to the reduction in a cost of a secondary battery.
In view of the above, various studies on a technique for reducing the amount of hot gas in a hot-gas drying furnace have been made. For example, the technique disclosed in the following Patent Literature 1 has been publicly known.
In the prior art disclosed in Patent Literature 1, in a drying step which is performed after applying a paste for an electrode, the temperature of a hot gas is adjusted in consideration of the amount of decrease in temperature caused by evaporative latent heat in order to offset the amount of decrease in temperature caused by the evaporative latent heat. Thereby, the amount of the hot gas is reduced while preventing the temperature from exceeding an allowable upper-limit temperature of a workpiece.